1. Is the Onkyo powerful enough to drive the 5.1 and then 2-4 speakers in the other room?
Yes, (for 2 speakers).
Your Onkyo is a 7.1 AVR. That means it has 7 built-in amplifiers. You can set up a 7-channel home theater room, (7.1), or you can set up a 5-channel theater room, (5.1), and use the other 2 channels to drive 2 speakers in Zone-2. The Onkyo has plenty of power.
Now, you're beginning to hear what most of us think about a theater with only ceiling speakers. It is not our favorite for music or tv/movies. If we had wires hanging out of the ceiling for speakers, most of us would put some kind of cover over them and go with traditional towers or bookshelves. Ceiling speakers can be fine for your background party music, but not so much for personal listening or tv/movies. That being said, if you're determined to go that route, people here can still offer suggestions.
Subwoofers are a critical element. They allow you to spend much less on speakers, and provide the impact to tv/movies that separates a theater from a TV room. You'll quickly draw a negative reaction if you talk about scrimping or minimizing your sub(s).
You don't have to spend a ton to get a good system, and you're wise to come here and ask BEFORE you spend your money. If your budget is limited, we'll suggest building a system a bit at a time rather than getting a sub-par system all at once. There are some really good speaker options for ~$200/pair.
Tell us what you're willing to do and how much you're willing to spend. We can help.
Oh... the "mild abuse" is all in fun. Don't take it personally.
